feat(telemetry): export the site script compile-cache eviction counter as a Meter (ScadaBridge#68)

SiteScriptCompileCache now records each eviction sweep on scadabridge.site.script_compile_cache.evictions
(ScadaBridgeTelemetry). It rides the existing ZB.MOM.WW.ScadaBridge meter, which is already in
SiteServiceRegistration.ObservedMeters, so no new allowlist entry is needed; a Host test pins that
the meter stays allowlisted.

tests/ZB.MOM.WW.ScadaBridge.Host.Tests/SiteLocalDbWiringTests.cs

@@ -245,6 +245,17 @@ public class SiteLocalDbWiringTests : IDisposable
         Assert.Contains(LocalDbMetrics.MeterName, SiteServiceRegistration.ObservedMeters);
     }
 
+    /// <summary>Verifies that the ScadaBridge application meter (carrying the compile-cache eviction counter) is exported.</summary>
+    [Fact]
+    public void Site_Telemetry_Allowlists_TheScadaBridgeMeter()
+    {
+        // ScadaBridge#68: scadabridge.site.script_compile_cache.evictions lives on this meter; an
+        // unlisted meter is dropped silently (ZbTelemetryOptions.Meters is an allowlist).
+        Assert.Contains(
+            ZB.MOM.WW.ScadaBridge.Commons.Observability.ScadaBridgeTelemetry.MeterName,
+            SiteServiceRegistration.ObservedMeters);
+    }
+
     /// <summary>Verifies that the Phase 2 tables are created in the consolidated LocalDb file.</summary>
     [Fact]
     public void Site_LocalDb_CreatesThePhase2Tables_InTheConsolidatedFile()

tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Scripts/SiteScriptCompileCacheTests.cs

@@ -253,4 +253,32 @@ public class SiteScriptCompileCacheTests
         Assert.True(cache.Count <= 16);
         Assert.True(cache.Evictions > 0);
     }
+
+    /// <summary>
+    /// ScadaBridge#68: evictions are exported as a Meter counter (not only the rate-limited Warning
+    /// log), on the ScadaBridge meter that the Host already allowlists for export.
+    /// </summary>
+    [Fact]
+    public void Eviction_IsExportedAsAMeterCounter()
+    {
+        long exported = 0;
+        using var listener = new System.Diagnostics.Metrics.MeterListener();
+        listener.InstrumentPublished = (instrument, l) =>
+        {
+            if (instrument.Meter.Name == ZB.MOM.WW.ScadaBridge.Commons.Observability.ScadaBridgeTelemetry.MeterName
+                && instrument.Name == "scadabridge.site.script_compile_cache.evictions")
+                l.EnableMeasurementEvents(instrument);
+        };
+        listener.SetMeasurementEventCallback<long>((_, measurement, _, _) => Interlocked.Add(ref exported, measurement));
+        listener.Start();
+
+        var cache = new SiteScriptCompileCache(16, logger: null);
+        for (var i = 0; i < 40; i++)
+            cache.GetOrAdd("return " + i + ";", typeof(ScriptGlobals), Ok);
+
+        // The instrument is process-wide, so a parallel test's evictions may add to it: bound from below.
+        Assert.True(cache.Evictions > 0);
+        Assert.True(Interlocked.Read(ref exported) >= cache.Evictions,
+            $"exported {exported} but the cache evicted {cache.Evictions}");
+    }
 }